Monkeys are an important part of the diet in many populations of Harpy Eagles, although in some cases no single species of monkeys is of primary importance. Cebid monkeys, the remains of most of which could not be identified to species, made up ca 37% of the prey remains of a large sample (approximately 83 individuals) of mammalian prey from a nest site in Guyana (Izor 1985). Similarly, cebid monkeys, of six genera, contributed 35% of the prey items of a sample of 10 nests in Amazonian Ecuador (Muñiz-Lopez et al. 2007).

Sloths also are a particularly important part of the diet in many populations of Harpy Eagle. In Amazonian Ecuador, for example, two-toed sloths (Choloepus) alone made up 29% of the prey delivered to a sample of 10 nests, with three-toed sloths (Bradypus) contributing another 5 of the identified prey (Muñiz-Lopez et al. 2007). At the nest in Guyana studied by Rettig (1978), Choloepus and Bradypus contributed 23% and 8% of a collection of 83 individual mammalian prey remains, a sample that was estimated to "represent a large majority of the kills made during the year of study" (Izor 1985). In a detailed observational study of two radio-tagged Harpy Eagles on Barro Colorado Island, Panama (both captive-bred subadults), Touchton et al. (2002) recorded that Choloepus and Bradypus combined accounted for just over half of prey captures for both individuals (52% and 54%, respectively).

Peres (1990) witnessed a successful attack by a Harpy Eagle on an adult howler monkey (Alouatta); the size of this monkey was estimated to be up to 6.5 kg in mass. Rettig (1978) similarly estimated the size of a howler monkey, half of the carcass of which was taken to a nest by a female Harpy Eagle, as 6.3 kg.

Behavior

The behavior of the Harpy Eagle is little-known, aside from observations at a few nests, and of a very few accounts of its foraging behavior. Judging by the prey that is brought to nests, Harpy Eagles often hunt in the canopy. The few direct observations of attacks on prey by Harpy Eagles are from the edges of the forest, such as along the edges of oxbow lakes and rivers (e.g., Eason 1989, Peres 1990). This may suggest that Harpy Eagles preferentially hunt along forest edges (Bierregaard 1994), although perhaps most observations of Harpy Eagle attacks are from forest edges because these situations provide longer sight lines for human observers.Harpy Eagles probably hunt most often by watching from perches in the canopy, then sally after prey, with a rapid and agile attack. In a successful attack observed by Peres (1990), the eagle "appeared rapidly from low over the canopy." Following a successful attack, the eagle often carries its prey to the top of a tree (Bierregaard 1994). If the prey is too heavy, it may be taken to a fallen tree.As might be expected from the significant differences in size between the sexes, male and female Harpy Eagles differ in the size of the prey that they capture. The reported size range of prey brought to a nest by the female member of a pair was up to 3.2 kg, whereas size of the prey provisioned by the male ranged from 0.5-2.3 kg (Rettig 1978). Similarly, when provisioning the nest, the sexes differ in the size of the pieces of prey brought to the nest. Females return to the nest with items that are estimated to weigh up to 4 kg, whereas the pieces brought by males are estimated to weigh only ca 1 kg.The Harpy Eagle rarely if ever soars (Thiollay 1984, Hilty and Brown 1986, Ridgely and Greenfield 2001, Hilty 2003). It may perch at forest edge or on a prominent site in an emergent tree (especially while sunbathing), otherwise it usually is found in the canopy.

Territoriality

Little known. Pairs have been reported nesting within 3 km of one another in both Panama and in Guyana, and within 5 km in Venezuela (Bierregaard 1994). The average distance between three simultaneously active nests in souteasern Peru was 7.4 km; the area occupied by each breeding pair was estimated to be 4300 ha (Piana 2007). In other studies the estimated home range of a pair of Harpy Eagles is as large as 25 x 25 km.

Sexual Behavior

Little known. Rettig (1978) witnessed nine copulations, in the nest tree, between a pair of adult Harpy Eagle, but did not observe any sign of a courtship display prior to copulation.

Social and interspecific behavior

Solitary, except when breeding.

Predation

No information.

Reproduction

The breeding of the Harpy Eagle now has been studied at several different sites, from Pamama south to Argentina (Fowler and Cope 1964, Rettig 1978, Chebez et al. 1990, De Lucca 1996, Muñiz-Lopez et al. 2007). The nest is in the crown of a tree, usually in a tree that emerges above the canopy; these trees may reach 50-75 m in height. Nests in Guyana and Brazil were placed 33-41 m above the ground (Bond 1927, Fowler and Cope 1964). Five nests in Madre de Dios, Peru ranged in height from 21-27 m, with a mean height of 24.5 m (Piana 2007). The nest site can be lower where the forest has a lower stature; four nests in Misiones, Argentina, were at 16-21 m above the ground, in trees that were only 20-30 m in height (Chebez et al. 1990, De Lucca 1996).In southeastern Peru, the number of principal branches in an emergent tree is the character most closely associated with nest site selection by Harpy Eagles; emergents with four or more prinicpal branches were the more likely to be used by Harpy Eagles for nesting (Giudice et al. 2007). Four out of six nests examined in southeastern Peru were in brazil nut trees (Berthollettia excelsa; Lecythidaceae); the two other nests both were in Dipteryx micrantha (Fabaceae) (Piana 2007). Elsewhere the nest often is in a Ceiba (Bierregaard 1994); for example, 8 out of 11 nest trees identified in Ecuador were in Ceiba pentandra (Muñiz-Lopez et al. 2007). Ceiba also has been reported a nest tree in Guyana, as has Mora excelsa (Fowler and Cope 1964). Other tree species reported as nest sites include species of Bombacaceae, Enterolobium contortisiliquum (Fabacese), and Mauritia palms (Bierregaard 1994, De Lucca 1996).The nest is a large, bulky stick platform; the dimensions of one nest in Guyana were 133 cm across, and 78 cm in height (Rettig 1978). The same nest may be used repeatedly. Rettig (1978), for example, studied a pair in Guyana in 1975 that was at the same site as a pair studied in 1960 by Fowler and Cope (1964). Both sexes contribute to nest construction. The nest primarily is composed of large sticks. These are supplemented with green sprigs; at a nest in Guyana, both sexes brought green sprigs to the nest before egg-laying, but after egg-laying commenced sprigs were provided primarily by the female (Rettig 1978).The usual clutch size is two, although a clutch of three has been reported. Regardless, in all or almost all cases only a single young is raised, since there are no observations of nests with two large nestlings.Incubation begins after the second egg is laid, and the incubation period is 56 days (Rettig 1978). Almost all of the incubation is performed by the female, as is the feeding of the hatchiling (Rettig 1978). The male provisions the incubating female periodically. At one nest in Guyana, the male brought prey to the nest about once a week during incubation, and about every 3.5 days during the first half of the nestling period (Rettig 1978). After bringing prey to the nest, the male incubated while the female ate (Rettig 1978). Later in the nestling period, when both adults were hunting, prey was brought to the nest an average of every 2.5 days (Rettig 1978).At one nest in Guyana (Rettig 1978), the nestling first attempted to stand at 26 days of age. By 36 days, the nestling could stand, and could walk, but only clumsily. Fledging occurs ca four and a half to six months after hatching. Following fledging, the juvenile remains within 100 m of the nest for up to 12 months. During this period the juvenile is dependent upon the adults for food. In a frequently-noted display (Fowler and Cope 1964, Rettig 1978), the dependent juveniles involves calling while flashing the partially-opened wings.Due to this slow pace of the development of the young, successfully nesting pairs can breed only during every second or third year.

Populations and Demography

Little known. The Harpy Eagle usually is considered to be "rare" (Hilty and Brown 1986, Ridgely and Gwynne 1989, Howell and Webb 1995, Schulenberg et al. 2007), but it also is not readily detected (Hennessey et al. 2003) and so its true status in many areas is not well known (Hilty and Brown 1986).